In a vehicle provided with an internal combustion engine, vacuum formed in an intake pipe of the engine is used for operating the brake booster of a vehicle brake device, various exhaust gas cleaning devices, air conditioning devices, vacuum meters, and the like. If each of the above-mentioned devices is connected, via a respective vacuum pipe, to the intake pipe, an increase in the manufacturing cost, as well as an increase in the complexity of the structure of the intake pipe, is inevitable.
Therefore, a vacuum union capable of transmitting a vacuum signal to the various devices from the intake pipe has been provided. Because the vacuum level required by the brake booster is relatively higher than the vacuum level required by other devices (in other words, the exhaust gas cleaning devices, the air conditioning devices, the vacuum meters, and the like), the vacuum union is comprised of a tubular body of a large diameter mounted to the intake pipe for connecting the intake pipe to the brake booster, and a branch pipe of a small diameter mounted to the tubular body for connecting the body to the other above-mentioned devices. An orifice, for adjusting the level of the vacuum signal to be transmitted to the brake booster, is arranged in the tubular body.
However, because the orifice is arranged upstream of the branch pipe with respect to the intake pipe in the known vacuum union, the level of the vacuum signal which is transmitted to the brake booster drops considerably when one of the vacuum-operated devices other than the brake booster is damaged, thereby opening the branch pipe to the atmosphere. This causes bad operations of the brake booster, which in turn may cause a very dangerous driving condition when the vehicle is being operated at a high speed.